dt-bindings: msm/mdp5: Add MDP5 display bindings

Add a new doc for DT bindings for platforms that contain MDP5 display
controller hardware. The doc describes bindings for the top level
MDSS wrapper hardware and MDP5 itself.

Add an example for the bindings as found in MSM8916.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>